Where are the horses?
Imagine a team of horses—fifty or a hundred or even more—pulling
your car down the highway! Actually, your car’s engine has that much
pulling power—the pulling power of a certain number of horses. And
that’s how its power is measured—in horsepower.
When engines were first used to run machines, people compared the power
of an engine with the work a horse could do. An engine-maker would say
that an engine could do the work of two horses, or pull as hard as a
four-horse team.
But this was only a very rough measurement. Some horses pull harder than
others. Until someone found a way to measure the power of an engine, no
one could be sure that an engine was strong enough to do a job.
James Watt, an inventor, made some important improvements in engines.
And he found a way to measure the real power of an engine.
First, Watt tested two strong workhorses to find out how much weight
they could pull. He found that a good horse could pull 550 pounds (247.5
kilograms) one foot (30 centimeters) in one second. Next he measured the
pull of engines. An engine that could do the same amount of work in one
second as one horse could do was said to have one horsepower.
We still measure an engine’s power this way. When we say that a car’s
engine has 150 horsepower, we mean that the engine can make a pull 150
times as strong as the one- horse pull James Watt measured. And the
engine in a car takes up much less space than 150 horses!
